Comparison of different salt separation and crystallization processes for zero sewage discharge
Release time：
2023-05-31
The salt separation and crystallization processes commonly used in zero sewage discharge include thermal salt separation, evaporation + freezing crystallization, and nanofiltration + separate evaporation and crystallization.
The salt separation and crystallization processes commonly used in zero sewage discharge include thermal salt separation, evaporation + freezing crystallization, and nanofiltration + separate evaporation and crystallization.
01 Thermal salt separation
The principle is to use the difference in solubility of sodium sulfate and sodium chloride at different temperatures to evaporate at variable temperatures.
Sodium sulfate is precipitated by controlling high temperature (80-90°C), and crystalline sodium chloride is precipitated by controlling low temperature (40-50°C).
Advantages: variable temperature crystallization, relatively simple process and equipment.
shortcoming:
(1) Theoretically, the quality of sodium sulfate and sodium chloride can meet the requirements of first-grade products, but it is found that it is difficult to achieve it from actual operation cases, and only one qualified product can be produced.
(2) The process control is difficult, because the solubility of the two salts varies with temperature in a small range, the control requirements for the crystallization of the elemental salt are very strict, and the circulation of the mother liquor is large. Due to the existence of other impurity ions in the water, the solubility of each salt is greatly affected, which will affect the stability of the quality of the crystallized salt.
(3) The requirement for the fluctuation of incoming water composition is relatively high, and the operating flexibility is poor.
(4) The ratio of sodium chloride and sodium sulfate is different, and the evaporation process changes greatly.
(5) The amount of discharged mother liquor is large. If the mother liquor is not treated, the resource recovery rate of crystal salt is low.
02 Evaporation + frozen crystallization
Taking advantage of the low solubility of sodium sulfate decahydrate at low temperatures (for example, at -5°C, the content of sodium sulfate at the eutectic point of sodium chloride and sodium sulfate is 0.71%, and the content of sodium chloride is 25.06%). Sodium sulfate is separated by freezing, and the frozen mother liquor (the mass ratio of sodium chloride and sodium sulfate reaches 35:1) is evaporated and crystallized at high temperature to obtain sodium chloride.
advantage:
(1) It has good applicability to concentrated brine with different ratios of sodium sulfate and sodium chloride, and has great operating flexibility.
(2) The nanofiltration membrane is better for the system tolerance of high hardness and high silicon.
(3) The equipment is relatively simple, and the investment and operation costs are not high.
shortcoming:
(1) The freezing temperature of the mother liquor is not well controlled, which has a great influence on the fluctuation of the composition of the mother liquor.
(2) All impurity ions or COD in the system will be enriched in the mother liquor, which will easily affect the quality of sodium chloride. It belongs to the salt separation process with better versatility and effect.
03 Nanofiltration + separate evaporation and crystallization
Using the principle of nanofiltration, the monovalent ions in concentrated brine are separated from the divalent ions.
The monovalent ions are mainly sodium chloride, containing potassium, nitrate and a small amount of sulfate, and relatively pure sodium chloride is obtained by evaporation and crystallization.
The divalent ion solution mainly contains sodium sulfate and part of sodium chloride, and most of the organic matter is enriched in the solution of divalent ions (advanced oxidation), which is evaporated and crystallized to produce anhydrous sodium sulfate.
advantage:
(1) It has high adaptability to the fluctuation of incoming water, and the water quality of the decrystallization unit is controlled by nanofiltration to be relatively stable, so as to ensure that the quality of crystallized salt is relatively stable.
(2) The process control is relatively easy and the degree of automation is high.
(3) It is beneficial to ensure the continuous and stable operation of production.
(4) Sodium sulfate and sodium chloride are completely separated, less mother liquor is discharged, and the recycling rate of crystal salt is the highest.
shortcoming:
The equipment investment is high, the service life of the nanofiltration membrane is short, and the operating cost is high.
It is more suitable for concentrated brine with high sodium chloride content, low COD and low hardness.
